Fix basepath root handling (#14756)

Fixes https://github.com/vercel/next.js/issues/14734

packages/next/next-server/lib/router/router.ts

@@ -23,19 +23,15 @@ import {
 const basePath = (process.env.__NEXT_ROUTER_BASEPATH as string) || ''
 
 export function addBasePath(path: string): string {
-  return `${basePath}${path}`
+  return basePath ? (path === '/' ? basePath : basePath + path) : path
 }
 
 export function delBasePath(path: string): string {
-  return path.substr(basePath.length)
-}
-
-function toRoute(path: string): string {
-  return path.replace(/\/$/, '') || '/'
+  return path.slice(basePath.length) || '/'
 }
 
 function prepareRoute(path: string) {
-  return toRoute(delBasePath(path || '') || '/')
+  return removePathTrailingSlash(delBasePath(path || '/'))
 }
 
 type Url = UrlObject | string
@@ -196,7 +192,7 @@ export default class Router implements BaseRouter {
     }
   ) {
     // represents the current component key
-    this.route = toRoute(pathname)
+    this.route = removePathTrailingSlash(pathname)
 
     // set up the component cache (by route keys)
     this.components = {}
@@ -443,7 +439,7 @@ export default class Router implements BaseRouter {
         method = 'replaceState'
       }
 
-      const route = toRoute(pathname)
+      const route = removePathTrailingSlash(pathname)
       const { shallow = false } = options
       const cleanedAs = delBasePath(as)
 
@@ -782,7 +778,7 @@ export default class Router implements BaseRouter {
       if (process.env.NODE_ENV !== 'production') {
         return
       }
-      const route = toRoute(pathname)
+      const route = removePathTrailingSlash(pathname)
       Promise.all([
         this.pageLoader.prefetchData(url, asPath),
         this.pageLoader[options.priority ? 'loadPage' : 'prefetch'](route),

test/integration/basepath/pages/link-to-root.js

@@ -0,0 +1,11 @@
+import Link from 'next/link'
+
+export default function Page() {
+  return (
+    <>
+      <Link href="/">
+        <a id="link-back">back</a>
+      </Link>
+    </>
+  )
+}

test/integration/basepath/test/index.test.js

@@ -312,6 +312,13 @@ const runTests = (context, dev = false) => {
     expect(pathname).toBe('/docs/other-page')
   })
 
+  it('should have correct href for a link to /', async () => {
+    const browser = await webdriver(context.appPort, '/docs/link-to-root')
+    const href = await browser.elementByCss('#link-back').getAttribute('href')
+    const { pathname } = url.parse(href)
+    expect(pathname).toBe('/docs')
+  })
+
   it('should show 404 for page not under the /docs prefix', async () => {
     const text = await renderViaHTTP(context.appPort, '/hello')
     expect(text).not.toContain('Hello World')